It is common practice to inspect work pieces subsequent to production on a coordinate positioning apparatus, such as a coordinate measuring machine (CMM), in order to check for correctness of predefined object parameters, like dimensions and shape of the object.
In a conventional 3-D coordinate measurement machine, a tool carrier is supported for movement along three mutually perpendicular axes (in directions X, Y and Z). Thereby, the tool carrier can be guided to any arbitrary point in space of a measuring volume of the coordinate measuring machine and the object is measurable with a measurement sensor arrangement (sensor chain) carried by the tool carrier.
In a simple form of the machine a suitable transducer mounted parallel to each axis is able to determine the position of the tool carrier relative to a base of the machine and, therefore, to determine the coordinates of a measurement point on the object being approached by the sensor. For providing movability of the tool carrier (probe head) a typical coordinate measuring machine may comprise a frame structure on which the tool carrier is arranged and driving means for moving frame components of the frame structure relative to each other.
For measuring surface variations, both measurement principles based on use of tactile sensors and of optical sensors are known.
In general, to provide a coordinate measuring machine with high measurement precision, its frame structure is therefore usually designed to have a high static stiffness.
In order to achieve a stiff and rigid machine design, the frame structure or at least parts of it, is often made of stone, such as granite.
For measuring an object automatically, a part program needs to be written in order to provide suitable guidance of the measuring probe according to the dimensions and the shape of the object. Such part program contains all necessary information about the object (work piece) and about the features to be measured. In addition, the sensor chain needs to be defined with the part program, i.e. what kind of measuring head, what kind of extension, what exact sensor and what stylus is to be added for fulfilling the given requirements of the measurement. Typically, some software tool help doing this selection in a more convenient way, but that operation generally stays a manual process.
Some general limitations are given with this process. When defining the sensor chain for the first time, the user has to select, particularly out of a very long list of items, which sensor elements are in use. This can be quite challenging, as often the exact sensor name is not known or if local names for some of the components are used.
Furthermore, there is always the possibility given to forget to set e.g. a stylus extension or a further component of the sensor chain. Finally, an overall risk is that the sensor chain definition will not be correct, leading to unwished crashes.
In addition, when later on re-using a given part program, the user needs to ensure the right sensor chain, i.e. each component of the chain, which is mounted on the CMM. Such operation may generate corresponding errors as mentioned above. As a consequence, if the sensor chain is not correctly defined in the measuring software, the wrong dimensions will be used, usually generating collisions between the sensor elements and the work piece or the CMM structure.
An approach for overcoming the problem of manually defining the sensor chain is the use of a known electronic identification system. An identification chip in every sensor component and a serial identification bus connecting all components to a master unit are provided, wherein each component which is attached is listed and thus the sensor chain is definable.
Disadvantageously, there however is no standard for that technology and only a few sensor components are equipped with such an electronic identification chip. In any case, a stylus never could be equipped with some electronics, what exclude them from such a system. Moreover, already existing sensor components without an electronic identification chip may not be provided with such chip and thus cannot be considered. Consequently, that quite complex electronic solution is expensive, is not standardized and cannot cover the complete sensor chain or sensor components without a chip.